Next-generation wireless communication systems may use high-frequency bands, for example, millimeter Wave (mmW), to support a high-volume data service. For a system using high-frequency bands, a communicable distance between a Mobile Station (MS) and a Base Station (BS) is reduced, such that a cell radius of the BS is also reduced, increasing the number of BSs that should be installed to secure a service area for the MS. As the cell radius of BSs is reduced and the number of BSs increases, the frequency of handovers increases due to movement of the MS between cells, increasing the amount of resulting signaling and resulting consumption of system resources.
The increase in the number of BSs increases requirements for the processing capacity of an access gateway that is connected with a BS to deliver data to the BS and to manage service information and authentication information for a user. Thus, in such a small-cell environment, a centralized network structure may be difficult to apply.
In a small-cell-based wireless communication system, a network structure is needed to increase a data rate while reducing a system load caused by frequent handovers. To solve problems caused by the small-cell structure, a distributed flat network structure has been considered. In a flat network, a small BS is used as a basic geographical unit for communication and several small BSs are grouped to operate as one virtual cell, a cluster.
If a MS have not transmitted or received a signal to or from a BS for a predetermined time, the MS operates in an idle mode for power saving. The MS in the idle mode wakes up in a predetermined cycle while not receiving a signal of the BS, to monitor whether a paging message sent thereto from the BS exists, and determines whether to transit to an active mode. The MS in the idle mode performs location update based on a timer or a paging group to inform a core network of a wireless communication system of its location. The core network recognizes a rough location of the MS according to the location update of the MS, and sends a paging message for the MS if necessary.
The paging group-based location update is performed when the MS moves to a BS of a paging group that is not a paging group to which the BS currently belongs. However, in a flat network structured in which numerous small BSs are directly connected to a higher-layer entity, exchange between many paging-related messages is needed when the MS enters the idle mode and relatively many BSs are included in one paging group.
Therefore, a need exists for a technique for efficient idle-mode operations of the MS in such a flat network structure.